The present invention is directed to a machine tool having three (3) axes of movement and a working element moved along these axes by linear motors. The working element is capable of moving in a work zone. The present invention is thus applicable to various machines of this type; but for sake of brevity, the invention will be described in connection to a cutting tool machine having a cutting tool. However, the invention is not to be limited as to only covering cutting tool machines. U.S. Pat. No. 5,368,425 discloses a linear motor-driven, cutting tool machine, and as present in numerous conventional screw-driven cutting tool machines, a vertical column is mounted on a slide to travel in a horizontal X-axis direction and a vertically-movable slide is slidable vertically along the column in the Y-axis direction. A spindle carrying a tool is mounted on the slide for sliding horizontally in a Z-axis direction at a location spaced above the X-axis and normal to both the X and Y axes, respectively. Each of these three axes is typically stacked one on another, resulting in the axis driving forces being offset from the spindle and cutting tool, e.g., a milling cutter, for cutting a piece of steel. This asymmetrical arrangement of the driving forces relative to the axis of the cutting tool results in deflections in the machine structure and inaccuracies in the cutting process. This is true whether the driving force is supplied by a servomotor and ball screw/nut combination or by way of linear motor drives, as in the '425 Patent. This asymmetrical arrangement results in: 1) structural deflections due to working forces or acceleration/deceleration forces, 2) in bending as experienced in the cantilevered load of the spindle slide and spindle, and 3) in less stiffness per unit of mass due to the large mass needed to achieve the necessary stiffness.
U.S. Pat. No. 5,368,425 discloses a cutting tool machine having its large upright column mounted for traversing in the X-direction along a large stationary, heavy base that supports the column. The driving force from the linear motors is applied only to the lower end of the column to propel it in the X-axis direction. The large stationary base provides the horizontal alignment of the respective X and Z-axis movements of the column and of the spindle slide and its tool spindle that travel vertically in the Y-axis direction along a vertical wall of the massive column. The spindle travels in a direction of the Z-axis and is carried on a cantilevered or overhung slide that travels along the vertical column wall. Thus, the machine alignment is predicated on the foundation remaining stable. If a corner of the foundation sinks or becomes misaligned, then the horizontal axes will be misaligned. Because of the space needed to support the column at its bottom end, and the size of the tool-carrying slide, the tool spindle and tool could not reach the bottom of a workpiece easily. To overcome this, the foundation required a pit to be dug to lower the spindle to reach a bottom portion of the workpiece to machine the same. Such foundation pits are expensive, as are the large and often custom-sized, foundations for the kind of machine tools illustrated in this patent. Further, adding to the size of the foundation and to the size of the machine, per se, are the covers for the ways which are bellow-shaped and which are located at the ends of the ways. These bellow-shaped covers are located at positions on the machine base at the ends of the column travel in the X-axis; and hence, add to the overall length footprint of the machine. The covers keep chips and metallic dust from the ways and from being magnetically attracted to the linear motor parts.
To move the massive column in the X-axis in the machine disclosed in the aforesaid patent, there is a pair of linear motors with one-half of each linear motor being mounted on the stationary base and the other half being mounted on the X-axis carrier supporting the column. The one-half of each of these two motors on the column adds to its overall weight, and thereby requires more linear motor thrust for the column to be moved along the X-axis. In addition to this linear motor weight on the column, there is an attractive, magnetic force, i.e., a normal magnetic force component which, in this instance, is a downward magnetic force between the coils and magnets of these linear motors, e.g., 24,000 lbs. of downward force that causes an increase in friction that must be overcome to traverse the column-supporting carrier along the X-axis. If the column weighs 8,000 pounds, and a normal 24,000 lb. attractive, magnetic force is present, the latter adds significantly to the mass that needs to be overcome. The linear motors apply the thrust only to the bottom of the column. To resist deflection of the unsupported upper end of the column as the lower end accelerates, the column is formed with heavy structural members that add to the weight of the column. Hence, the linear motor force required to move the column at accelerations of one G or more is increased significantly from the force needed if the column were not driven only at its lower end, and if large no such attractive, magnetic, normal force was present.
Generally speaking, at the present time, it takes about one pound of thrust from the linear motor to move about one pound of mass. The larger the thrust needed, the larger linear motor weight that must be added to the column. Stated differently, the effect is cumulative because the more force needed to be obtained from the linear motors, the heavier the linear motor is in weight, e.g., the coils on the column carrier and this weight increase adds more weight requiring more thrust.
In the linear motor cutting machine, disclosed in the aforesaid patent, the massive column is constructed of lightweight materials and has a skeletized structure in order to reduce its weight; and thereby, reduces the linear thrust force needed to accelerate it rapidly and to reduce deflection of the upper end of the column relative to the lower end of the column, which is being propelled by the linear motors. The column is formed with an aluminum braced, trapezoidal-shaped frame and with an aluminum skin covering the skeleton frame and with a reinforcing ladder of frame elements thereon to add rigidity and stiffness to the column. Because one is machining metals, a high degree of stiffness is needed for the column which carries the cutting tool slide and spindle in order to obtain the precision needed for the cut workpiece surfaces.
In the aforesaid patent, the spindle-carrying slide was overhung or cantilevered on the column to move vertically along a Y vertical axis. The column carried on three sides thereof, permanent magnets for three (3) linear motors used to accelerate and decelerate the spindle slide. The stator coils of the motors were on three sides of this slide and located closely adjacent to and traveling along the three, associated vertical rows of permanent magnets on the column. Three linear motors were needed to accelerate and decelerate the spindle slide vertically, and these three linear motors generated normal attractive, magnetic forces directed inwardly along three sides of the column. The spindle-carrying slide was cantilevered on the column and the weight of the ram and spindle as well as the applying of the thrust only to the side of the slide adjacent the column could cause bending and deflection and sufficient structural members had to be used in the slide to offset any such bending or deflection. The three linear motors not only applied their thrust to only one side of the slide adjacent the column wall, but these linear motors also increased the friction between the spindle slide and the column that had to be overcome. Thus, if 18,000 lbs. of normal attractive, magnetic force existed between the column and the ram, this increased frictional force generated thereby had to be overcome to accelerate the spindle slide in the Y-axis direction and the slide structure had to be reinforced with structural members to resist bending and deflection of the remote end of the cantilevered slide as the motor thrust was applied to the near side of the slide at the column wall.
The size and weight of this spindle slide were quite large in order to support the three rows of coils for the three Y-axis motors and to provide a stiff, cantilever support for the slide and spindle thereon to provide the cutting tool with the necessary stiffness against the workpiece. The spindle slide, illustrated in this patent, had a ribbed structure that was quite large in cross-section and carried an outer aluminum skin about the ribs. The large size of the spindle slide meant that the tool could not be brought down as low as desired relative to the workpiece because the spindle slide's height below the spindle limited the amount of downward travel of the spindle.